The present invention relates to a process for producing transparent gels having improved mechanical and sorption properties.
Hydrophilic gels derived from 2-hydroxyethyl methacrylate have been found to be useful in a number of medical applications, i.e., as a material for gel filtration, etc. An equilibrium sorption of these gels is, however, maximally only 40 to 45% water and the gels have a relatively low mechanical strengths. If the content of the water in the polymerization medium is increased above this limit, the degree of equilibrium swelling can be raised, however, the gels then become heterogeneous and possess poorer mechanical properties. In addition, gels based on 2-hydroxyethyl methacrylate have an inherent low selectivity. There has been mentioned the possibility to modify the gels by addition of a small amount of another monomer, as for example by the addition of up to 20% of methacrylamide (Czechoslovak Pat. Specifications Nos. 109,159 and 133,433) these additions, however, only slightly influence the properties of the basic gel. Combinations of 2-hydroxyethyl methacrylate and acrylamide have been described in (British Pat. Specification No. 829,565), however, these copolymers swell very heavily in water, are cohesionless and therefore do not have much practical importance.
It has now been found, that the addition of a greater amount of methacrylamide or the like changes essentially, and in some respects even unexpectedly, the properties of the gel. The degree of equilibrium swelling in water may be enhanced more than twice as much when using copolymers with methacrylamide, without, however, deteriorating the mechanical properties thereof. The copolymers with for instance, methacrylamide, have demonstrably better mechanical properties (modulus, strength, elongation at break) than the homopolymers at the same degree of swelling. At the same time, their selectivity to ions increases. It has unexpectedly been found that the introduction of the amide unit gives the gel a special property, which includes the unexpected substantial improvement of the mechanical properties by a heat treatment of the swollen copolymers of for instance, 2-hydroxyethyl methacrylate with methacrylamide or acrylamide, which properties cannot be achieved with the corresponding homopolymers.